A bacteriophage is a virus that infects bacteria. Bacteriophage T4 infects E.coli bacteria. T4 invasion begins by touching the bacterial cell wall and injecting viral DNA through its tube-like queue (purple) into the cell. A 'DNA packaging machine' (middle and right panels) between the 'head' bacteriophage' and 'queue' (green, yellow, blue tips) confirmed double-stranded DNA (middle panel, red) at hand.